This invention relates generally to systems for production of a gas from a mixture of an oxygen-containing gas stream and a hydrocarbon-containing gas stream. An example of where this invention has utility is systems for industrial production of ethylene oxide.
The chemical compound ethylene oxide (chemical formula C2H4O) is an important industrial chemical used as an intermediate in the production of ethylene glycol (the main component of automotive antifreeze) and other chemicals. Ethylene oxide is also used as a sterilant for foods and medical supplies. It is a colorless flammable gas at room temperature, and can be cooled and stored as a liquid.
Ethylene oxide first achieved industrial importance during World War I as a precursor to both ethylene glycol and the chemical weapon mustard gas. In 1931, Theodore Lefort, a French chemist, discovered a means to prepare ethylene oxide directly from ethylene and oxygen, using silver as a catalyst. Since 1940, almost all ethylene oxide produced industrially has been made using this method.
In current industrial processes, ethylene oxide is produced when ethylene (CH2═CH2) and oxygen (O2) react on a silver catalyst at 200-300° C. showing large Ag nanoparticles supported on Alumina. Typically, chemical modifiers such as chlorine are also included. Pressures used are in the region of 1-2 MPa. The chemical equation for this reaction is:CH2═CH2+½O2→C2H4O
In ethylene oxide production systems, a gas mixer is used to mix the hydrocarbon and oxygen gas streams just upstream of the reaction chamber where the silver catalyst is present. The gas mixer is typically constructed in the form of a vessel or pipe. The vessel includes an inlet manifold for each of the two gases. The vessel is sometimes constructed with a main outer pipe containing the hydrocarbon-containing gas stream and internal concentric tubes or “fingers” which contain the oxygen stream. Mixing occurs at the point where the internal tubes end, where the oxygen gas flowing out of the fingers meets the main stream of hydrocarbon-containing gas flowing in the outer tube. This basic design is described in U.S. Pat. No. 3,706,534.
The art has long recognized that there is a risk of ignition of a hydrocarbon-containing gas stream (e.g., a stream of gas containing for example ethylene mixed with other hydrocarbon gases) at the point where it is combined with an oxygen gas in a gas mixer. Ignition can occur when a particle (e.g. a piece of rust or pipe scale) entrained in the hydrocarbon or oxygen gas stream strikes a metallic surface in the mixer, e.g., the wall of the mixer, thereby producing a spark. If the spark occurs in the hydrocarbon stream in an area of high concentration of oxygen, e.g., at, or close to, the point of mixing of the two gas streams, ignition can occur. The ignition damages the gas mixer and also requires an interrupt of production to suppress the ignition and allow the gas mixer to cool before recommencing production. The flammable region is confined to the mixing zone of the two gases. The hydrocarbon gas as well as the reactor feed blend are below the lower O2 flammability limit—i.e., too rich to burn.
The art has devised a variety of gas mixer designs. Some of the designs are specifically directed to reducing the risk of ignition of hydrocarbon and oxygen gas stream. The known prior art includes the following patent documents, in addition to the above-cited '534 patent: U.S. Pat. No. 4,573,803; U.S. Pat. No. 3,702,619; U.S. Pat. No. 4,256,604; U.S. Pat. No. 4,415,508; U.S. Pat. No. 6,657,079; U.S. 2003/0021182; U.S. Pat. No. 3,518,284; U.S. Pat. No. 4,390,346; U.S. Pat. No. 3,237,923; U.S. Pat. No. 3,081,818; U.S. Pat. No. 2,614,616 and U.S. Pat. No. 6,840,256.
Oxygen supply lines contain particles that can cause ignition hazards. The hazards include sand, dust, metal, and partially oxidized metal particles, although other inert contaminants may pose hazards. At one extreme, impact of large particles, about 100 to 2000 microns, may cause the metal that forms the oxygen piping, valves, and flow control equipment to ignite. At the other extreme, small particles of about 5-1000 microns may cause ignition in the mixers of partial oxidation processes such as ethylene oxide and glycol, or related partial processes using high purity oxygen. It is common practice to use strainers in oxygen supply lines to remove large particles. These do not remove the small particles that can cause mixer fires. Smaller particles down to about 10 microns may be removed using fine filters, but this creates other problems. The filters are liable to clog and are at risk of ignition due to spontaneous ignition or frictional heating, which can cause a fire in the oxygen supply line. The latter is typically caused by poor maintenance or loosening of components over time, creating rubbing of the metal components of the filter.
Additionally, the current practice of filtration and strainers accumulates and concentrates contaminants in the device. This also necessitates periodic cleaning and removal of particulate that has been captured. Pleated metal, ceramic, or mineral wool filter elements collect the particulate. Often the filter housing will contain a number of filter elements operating with parallel flow paths. When the filters collect sufficient material, current practice is to briefly shut the plant down to clean the filter elements. The concentrated particulate can be a source of kindling material for an oxygen fire. These manual operations expose people to hazards. In addition, removing particulate of smaller sizes, such as in the range of 5-30 microns requires more complex and costly filtration devices.
Other prior art of interest include the following patents directed to wet scrubbing technology: U.S. Pat. No. 6,231,648; U.S. Pat. No. 4,012,469; U.S. Pat. No. 5,178,654 and U.S. Pat. No. 5,250,267. Wet scrubbers have been used heretofore in several applications, including mining, semiconductor fabrication, and others, such as for example to remove coal dust, toxic or flammable gases or other contaminants, e.g., sulfur compounds, from a supply of air which is to be released into the environment. To the knowledge of the inventors, wet scrubbing technology has not been previously adopted in ethylene oxide or related production systems.
This disclosure solves a long-felt need in the art for a solution to the problem of removal of particles down to about 5 microns size in an oxygen supply line, while not concentrating the particles, and avoiding problems with clogging or ignition of filters in the oxygen supply lines. Furthermore, production systems using the features of this disclosure avoid the need for a process shutdown to manage the accumulated particulate in screens or filters. Furthermore, it provides for methods of eliminating particulate matter without accumulating material that may be a source of material for a fire in the oxygen supply line.